1. Field of the Invention
This invention relates to an insecticidal composition comprising, as an active ingredient, an optically active isomer of .alpha.-cyano-3-phenoxybenzyl 2-(4-chlorophenyl)isovalerate (of the acid moiety or both the acid and alcohol moieties of the compound) represented by the formula (I) ##STR1## which exhibits a remarkable insecticidal activity without any appreciable toxicity to plants as well as to mammals.
The insecticidal composition according to the present invention is very useful since it can be used in farms and gardens for sanitary use, in grain stores and in forestry.
2. Description of the Prior Art
At present, an extract of pyrethrum flowers (containing pyrethrin) and allethrin which is a synthetic analog of the active component of the extract of pyrethrum flowers are generally used as rapid active insecticides having no appreciable toxicity to human or animals. However, in spite of their excellent utility, the use of pyrethrum flower extract is limited due to its relatively high production cost. Moreover, these compounds are unstable to light and, thus, they tend to rapidly lose their activity in the field when used in gardens.
As a result of synthesizing various compounds and optically active isomers thereof and testing their biological activities, it was found that the optically active isomers of the compound represented by formula (I) above showed excellent insecticidal activities in respect to insects such as flies and mosquitoes which are harmful to hygiene and agriculturally injurious insects such as plant-hoppers, leaf hoppers, aphids, boll worms, diamondback moths and army worms, while having no phytotoxicity on crops and plants and low toxicity to human and animals. It was also found that these isomers can be produced comparatively cheaply.
It is well known that the racemate .alpha.-cyano-3-phenoxybenzyl 2-(4-chlorophenyl)isovalerate has an insecticidal activity as disclosed in U.S. Pat. No. 3,996,244. However, when the present inventors synthesized the optically active isomers on the acid and alcohol moieties of this compound and supplied them for biological assay, they discovered that the ester of the S(+) acid had a stronger insecticidal activity than the racemate. In particular, even stronger activity was shown by the ester of the S(+) acid and the S(-) alcohol. Moreover, in the case of many crops and plants, racemic .alpha.-cyano-3-phenoxybenzyl 2-(4-chlorophenyl)isovalerate exhibits a phytotoxicity causing partial chlorosis of the new shoots. However, with the ester of the S(+) acid and racemic alcohol and the ester of the S(+) acid and the S(-) alcohol discovered by the present inventors, the chlorosis was found to be low enough to cause no phytotoxicity at practical concentrations. Thus, the inventors found that these isomers are extremely beneficial as insecticides. The fact that the optically active isomers consisting of this specific combination of the acid moiety and the alcohol moiety simultaneously have extremely strong insecticide activity and no practical phytotoxicity to crops and plants is novel and a completely unexpected finding.